The present invention relates to a subband coding system for image data, and more particularly to a subband coding method and encoding/decoding system which are suitable for the band compression of luminance signal in a high definition television set (HDTV).
A known subband coding system of images is disclosed in U.S. Pat. No. 4,817,182. With reference to FIG. 1, which is a diagram of a conventional subband encoding system of image data, the system is constituted such that digitized image signals [X(m,n)] are analyzed into low-band and high-band signals in the row through filters 101 and 102 of a primary analyzing circuit 100, thereafter sampled by down-samplers 103 and 104. The sampled signals are analyzed into low-band and high-band signals in the column through filters 105, 107 and 106, 108, respectively, and thereafter sampled by samplers 109, 110 and 111, 112, thereby obtaining primary analysis signals Y11, Y12 and Y21, Y22. The primary analysis signals Y11, Y12 and Y21, Y22 are coded by encoders 31, 32 and 33. The primary analysis signal Y11 of low-band is analyzed into low-band and high-band signals and sampled in the row at the secondary analyzing circuit 200 through the same procedures as those in the primary analyzing circuit 100, and then analyzed again into low-band and high-band signals in the column and thereafter sampled in order to obtain secondary analysis signals Y11-11, Y11-12, Y11-21, and Y11-22. The secondary analysis signals Y11-11, Y11-12, Y11-21 and Y11-22 are then coded by the encoders 34, 35, 36 and 37 respectively.
Thus, since three component signals of high-band are coded among the primary analysis signals of four components which have been analyzed at the primary analyzing circuit 100 in the horizontal and vertical directions and one component signal of low-band is coded after obtaining a second analysis signal of four components which have been analyzed at the second analyzing circuit 200 into horizontal and vertical directions, seven component signals are coded in total.
Referring to FIG. 2 which is a diagram showing the conventional subband decoding system, the signals are recovered through the procedures in inverse order of those in FIG. 1. That is, the output signals which have been coded by the encoders 34, 35, 36 and 37 of FIG. 1, are recovered to signals Y'11-11, Y'11-12, Y'11-21 and Y'11-22 at the secondary signal recovering circuit 500 through decoders 44, 45, 46 and 47, respectively and pass through up-samplers 501, 502, 503 and 504 and filters 505, 507, 506 and 508, respectively, and then added with the outputs of the filters 505 and 507, 506 and 508 through adders 513 and 514. The output signals of the adders 513 and 514 pass through the up-samplers 509 and 510 and filters 511 and 512, respectively and are added by the adder 515 and then recovered to a signal Y'11. Recovering signals Y'12, Y'21 and Y'22 are sought by decoding the outputs of the encoders 31, 32 and 33 of FIG. 1 by the decoders 41, 42 and 43. The recovering signals Y'11, Y'12, Y'21 and Y'22 which have been sought by the secondary signal recovering circuit 500, are recovered at the primary signal recovering circuit 600 in the vertical direction through the same procedures as those in the secondary signal recovering circuit 500 and then recovered in the horizontal direction, whereby obtaining image data [X'(m,n)].
FIG. 3 is an explanatory view illustrating the continuous analysis with respect to the frequency band according to the conventional subband coding system of images. As shown in FIG. 3, when image data is bandpass filtered in the horizontal direction W1 and thereafter in the vertical direction, four component primary analysis signals Y11, Y12, Y21 and Y22 are obtained and when the central signal Y11 is analyzed in the horizontal and vertical directions, signals Y11-11, Y11-12, Y11-21 and Y11-22 are obtained, whereby it is possible to obtain seven component analysis signals Y11-11, Y11-12, Y11-21, Y11-22, Y12, Y21 and Y22 through one analyzing procedure.
However, in such a conventional subband coding system there has been a disadvantage in that a uniform analysis with respect to the frequency band is difficult because the frequency band of input signals [X(m,n)] is analyzed orthogonally, i.e., in both the horizontal and the vertical directions and one signal of low-band among the analyzed four components is analyzed into four component signals to obtain seven component analysis signals and also one signal of low-band is analyzed through the same procedures, resulting in the analyzing procedure being repeated two times, thereby causing the low-band component to have a very small frequency band.